Freezers wherein a helical conveyor belt section is wound in tiers around a rotating cage, oftentimes termed "spiral freezers", have become commercially favored because a relatively long length of conveyor belt can be provided for freezing food products or other products while occupying a relatively small amount of floor space, particularly as compared to commercial tunnel freezers wherein a linear conveyor belt is employed.
Various different types of such spiral freezers have been developed. For example, U.S. Pat. No. 4,739,623 shows such a freezer wherein liquid nitrogen is sprayed or otherwise brought into contact with the food products at a lower location near the cabinet inlet, and the vapor is circulated generally horizontally past the food products being carried by the belt as they rise toward an upper outlet from the cabinet. U.S. Pat. No. 4,356,707, in FIGS. 10-12, shows a spiral freezer of this general type wherein CO.sub.2 injectors are located in corner regions of the cabinet to inject CO.sub.2 snow and cold vapor and induce additional vapor flow generally horizontally and in a direction concurrent with the movement of the food products along the helical path. U.S. Pat. No. 4,324,110 shows a cryogenic food freezer of this general type wherein liquid CO.sub.2 is discharged countercurrently into streams of moving gas or vapor from fans to effect rapid vaporization of the injected CO.sub.2. U.S. Pat. No. 3,733,848 shows a food freezer of this general type wherein spray nozzles inject CO.sub.2 into discharge streams from vertically elongated blowers having vane-carrying squirrel cage rotors which rotate about vertical axes. U.S. Pat. No. 4,480,535 illustrates a food freezer of this general type wherein the pattern of gas flow within the freezer is primarily vertically through an open mesh or highly porous conveyor belt. U.S. Pat. No. 3,938,651 shows a freezer of similar construction wherein an interior cage is not driven but instead the conveyor belt itself is driven through an alternative linkage arrangement.
It is felt that freezing of food products is typically accomplished by heat transfer to the colder gas that is being circulated past the food products, although some heat may be withdrawn by removal to a vaporizing cryogen at its surface. Accordingly, the movement of the gas and its velocity become important to accomplishing efficient freezing of the food products. Likewise, the length of time during which the food products are exposed to the circulating cold gas is also important, and typically 10 or more tiers of belt are provided in the helical section.
In general, it has been found that the freezing of different food products presents different problems, and in order to accomplish the efficient freezing of different food products, it is often necessary to be able to make adjustments in gas flow and sometimes in overall time of exposure to the cold gas. Especially difficult freezing problems are presented by unwrapped and warm foods which tend to rapidly dehydrate until a solid envelope is created by the solidification of the surface, as for example, by the formation of an icy crust therealong. Likewise, the more rapidly surface solidification is accomplished, the less is the weight loss which occurs from the product and the fewer are the moisture or frost-related problems which must be compensated for within the freezer itself. However, once surface freezing has occurred, subsequent freezing becomes progressively more difficult because heat transfer must then be effected through the frozen skin, slowing the overall process.
As a result of the foregoing, it has been found that spiral freezers having a substantially uniform flow of air throughout the entire chamber, depending upon the character of the food products being frozen, will often have insufficient gas flow in one region, for example a lower region where initial freezing is occurring, and an excess of gas flow in another region, for example, an upper region wherein equilibration of final freezing is being accomplished. Moreover, the benefits achieved by overcoming such inefficiencies are much greater at colder freezer temperatures, i.e., when cryogenic or other freezers operating at temperatures of about -30.degree. F. or below are concerned. In addition, it has been found that uniformity of cooling radially across the belt may be difficult to accomplish. U.S. Pat. No. 4,078,394 shows a spiral freezer wherein some effort was made to try to adjust the flow of cold gas through various regions by employing a driven center cage in the form of a drum of circular cross-section having both its axial ends open and having a varied perforation pattern in its sidewall wherein the holes along the bottom edge are the largest and progressively decrease in size to the smallest size at the upper edge. Gas is sucked from the interior of the drum by a motor-driven fan and is discharged past a plurality of cryogen injectors in the top wall of the freezer. Various inefficiencies result from such a freezer design, and accordingly, improved solutions to the foregoing problems continued to be sought.